A computational model ofPseudomonas syringaemetabolism unveils the role of branched-chain amino acids in virulence expression at the early stages of Arabidopsis colonization

Abstract

ABSTRACTLeaf mesophyll-colonizing bacterial pathogens infect their plant hosts by adjusting their metabolism to the leaf mesophyll environment. Soon after the inoculation of naïve, susceptible plants, the model bacterial pathogenPseudomonas syringaepv.tomatoDC3000 (PstDC3000) expresses virulence factors that suppress plant immunity, a requirement to produce robust infections. However, if plant immunity was elicited with Microbe-Associated Molecular-Patterns (MAMPs) prior to bacterial inoculation,PstDC3000 slows down virulence expression and only produces symptomless mild infections. To understand how bacterial metabolism adapts to these two contrasting conditions, we created iPst19, anin silicoensemble of genome-scale metabolic reconstructions. Constraining thein silicogrowth of iPst19 within planta PstDC3000 gene expression data revealed that sugar catabolism is highly active in bacteria that have been inoculated in mock-treated plants. In contrast, branched-chain amino acids (BCAAs) catabolism is highly active in bacteria that have been inoculated in MAMP-pretreated plants. Bacterial growth and gene expression analysis showed that BCAAs suppress virulence gene expression without affecting bacterial growthin vitro. In planta, however, BCAAs suppress the expression of virulence genes at the early stages of the infection and significantly impair leaf colonization of the host plantArabidopsis thaliana. While the overexpression of the conserved bacterial leucine-responsive transcriptional regulatorLrpinduced the expression of virulence genes, its downregulation had the opposite effect, suggesting that BCAA-free Lrp induces virulence while BCAA-Lrpdoes not. Overall, our data provide mechanistic connections to understand how plant immunity impactsPstDC3000 metabolism and virulence, furthering our understanding of bacterial pathogenesis and plant disease.